The invention relates in general to travelling wire EDM apparatus wherein an electrode workpiece is cut by electrical discharges by means of an electrode tool in the form of a wire, and more particularly the invention relates to a travelling wire electrode rethreading device.
In travelling wire EDM apparatus the electrode wire, longitudinally displaced through the machining zone formed between the electrode wire and the electrode workpiece, is stretched between two guide members, the guide members being displaced laterally relative to the electrode workpiece according to a predetermined cutting path. At the end of a machining pass, the wire is cut to permit removing one of the machined parts and, prior to machining the next part, the wire must be rethreaded through a pre-drilled hole previously formed in the workpiece.
Known devices and methods for effecting automatic rethreading of the wire through the pre-drilled hole consist in, after cutting-off the wire end, pulling the wire through the pre-drilled hole or pushing the wire through a sleeve disposed in the pre-drilled hole and, subsequently, tieing together the two ends of the wire. Such known arrangements are complicated, they require a large number of consecutive steps, and the mechanisms accomplishing those steps are cumbersome and costly.
The disadvantages of the prior art are eliminated by the apparatus of the invention, wherein the electrode wire is threaded consecutively through a first zone in which it is straightened, and through a second zone in which machining is effected. The apparatus of the invention particularly provides means disposed in the first zone for cutting-off the wire in the lower portion of the first zone, and for pushing the straightened end of the wire through the second zone, disposed in alignment with the first zone, until the end of the wire is captured by a wire conveying mechanism disposed downstream of the second zone.
Feeding the wire from top to bottom, first through the zone where it is straightened and cut-off, and subsequently through the machining zone and into an ejection zone, permits to rethread the wire with high precision without requiring guiding the wire and without the necessity of connecting and tieing the two wire ends.
The many objects and advantages of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawing wherein: